cPanel creates a DNS zone for your domains once they are created from cPanel or WHM. You can modify the generated DNS zones from WHM.

I've corrected some syntax errors for Whistlers7.com.au.

If you need to forward email of a domain outside of the server you just need to modify the MX record and replace it with the remote email server hostname. Please note that IP addresses are not allowed for MX records.

You don't need to do A entries for mail subdomains unless your remote mail server has a web interface and you are going to access it using mail.domain.com.